FRESNO, Calif. (AP) -- Nitrate contamination of drinking water is a pervasive problem in California's agricultural heartland and is bound to intensify in the coming years, according to a University of California, Davis study released Tuesday.

The study, ordered by the state Legislature, shows chemical fertilizers and livestock manure are the main source of nitrate contamination in groundwater for more than 1 million Californians in the Salinas Valley and parts of the Central Valley.

While nitrate problems have been known for decades, the study offers a comprehensive assessment of how many people are exposed and identifies solutions and costs.

"In the near future, this problem is going to persist and is likely to get worse," said Thomas Harter, UC Davis groundwater hydrologist and the study's lead author. "Even if we were to eliminate all the sources of nitrate that we have today, we would still be dealing with this issue."

Nitrates are found naturally in some foods, but scientists have linked high levels of nitrates to "blue baby syndrome," reproductive disorders and cancer. Infants who drink water that exceeds the nitrate standard could become seriously ill and die, according the U.S. Environmental Protection Agency. State health officials say they don't track illnesses associated with nitrates.

The study -- covering the Salinas Valley and Fresno, Tulare, Kings and Kern counties -- concluded half of the 2.6 million people in those areas live in communities where raw drinking water sources have registered nitrate levels exceeding the standard. Many of those communities blend or treat their water, drill a new well or provide another alternative source, passing on the extra costs to rate payers.

Some wells in Fresno, population 500,000, have exceeded the nitrate standard. The city put a number of out of service, expanded its use of surface water and blended water from active contaminated wells with that in other wells.

The study also found about one in 10 people in the study area rely on untreated groundwater that may exceed the nitrate standards. Most are residents of small, poor agricultural communities which cannot afford to treat the water or offer alternatives.

"I grew up with contamination in my water and to think that I have to put in extra money just to make sure my child is going to be OK, it really bothers me," said Jessica Sanchez, a pregnant resident of East Orosi, an unincorporated farmworker community of 500 in Tulare County, where the two existing wells regularly exceed the safe standard for nitrates.

Sanchez, a college student, said her family pays $60 a month for contaminated water while spending at least that amount on bottled water for cooking and drinking.

If nothing is done, the study concludes, the financial burden on many agricultural communities could increase. By 2050, nearly 80 percent of the population -- about 2 million residents -- in the study area could have nitrate contamination exceeding the state standard.

"It shows how huge the scope of the problem is," said Laurel Firestone, co-director of the Community Water Center, a nonprofit advocating for safe drinking water in the Central Valley. "We need our drinking water sources, we rely on them daily. This contamination has been out of sight, out of mind for too long."

Cleaning up polluted aquifers would be too difficult, the study concludes. Improved farming practices and water blending, treatment and alternative water sources are more cost effective.

But treatment, Harter said, is very expensive, as is drilling a new or deeper well. Most at-risk communities like East Orosi don't have the means to make changes.

The state and regional water boards should assist those communities, Harter said, by providing legal and technical support and funding for solutions such as hooking up to larger communities for alternative water sources.

The study estimates addressing current nitrate contamination will cost the state $20 million to $35 million per year. The study proposes a fertilizer tax which would be used by affected communities to mitigate for nitrate contamination. Another funding option is water use fees from affected residents.

The study found that nitrate leaching from agricultural land is responsible for 96% of current groundwater contamination. And while fertilizer use has leveled off in recent years, the amount of dairy manure has increased, making for a net increase over the past decade in nitrates loaded into the ground.

That means contamination of drinking water will increase in future years, Harter says, because it takes time for nitrates to migrate.

California farmers say they have worked to change their farming practices to address the problem.

"There's a lot of energy all over the country on this issue and there's a host of people developing new technologies," said Hank Giclas, senior vice president of Western Growers.

Bob Martin, general manager of Rio Farms in King City, said he tests to find out how much nitrate the soil has and how much fertilizer, if any, it needs. This allows him to target the timing of the fertilizer and to reduce the amount used, Martin said.

His farm also spent about $11,000 last year on soil moisture probes used during drip irrigation. The sensors showed the farm was over-irrigating certain crops, meaning more nitrates were moving beyond the root zone and into the groundwater.

"We want to be good stewards of the land and we're doing everything we can," Martin said.